This invention relates to optical glass and an optical glass element manufactured by the optical glass.
It is a recent trend that a wide variety of image pickup devices have been developed and have been manufactured in a mass production fashion. In the meanwhile, it is to be noted that an optical system and an optical element, such as an optical lens are indispensably used in every one of the image pickup devices.
In order to mass produce such image pickup devices, such an optical element should be also precisely and quickly manufactured in a mass production fashion. In addition, the optical element must be light in weight because each image pickup device becomes small in size and light in weight.
Under the circumstances, proposals have been made to manufacture an optical element by the use of a press forming method which presses a glass preform by a pair of dies into an optical element, such as a spherical lens or an aspherical lens. In such a press forming method, neither a grinding process nor a polishing process is needed after the glass preform is pressed. Accordingly, the press forming method is effective to manufacture the glass element in the mass production fashion.
However, a recent requirement is also directed to an optical element which is more precisely pressed in shape and lighter in weight than a conventional optical element. For this purpose, improvements have been made about optical glass which has various glass compositions and which has low sag points and low specific gravities. This is because the optical glass can be easily press formed and becomes light when it has a low sag point and a low specific gravity.
Under the circumstances, optical glass of a P.sub.2 O.sub.5 --ZnO--R.sub.2 O (R:alkaline metal oxide) system has been disclosed in Japanese Patent Unexamined Publication Nos. Hei 2-116642 (namely, 116642/1990), Hei 3-40934 (namely, 40934/1991), and Hei 4-231345 (namely, 231345/1992). Each optical glass proposed in the above-mentioned publications has a sag point which is equal to or lower than 450.degree. C. However, such optical glass is not light in weight enough for responding to the recent requirement because ZnO which is comparatively heavy in atomic weight is included a lot in each glass material as an essential component.
Alternatively, disclosure is made in Japanese Patent Unexamined Publication No. Hei 4-325437 (namely, 325437/1992) about optical glass which has a transformation temperature lower than 325.degree. C. and which includes Cl or F, as an anion component. Addition of halide of Cl or F to optical glass serves to moderately lower the transformation temperature. However, it is noted that such Cl or F is volatile from the optical glass during a melting process of the optical glass. Volatilization of Cl or F makes it difficult to stably manufacture the optical glass which has a desired glass composition.